Clothes rigging/movement issues 2.79
Look up weight painting, its much more intuitive than creating vertex groups. I would recommend DanPro's videos at Youtube. They are based on Rigify, but the weight paint section applies.

Also, there is an issue with importing, since the character's default position is lying in his back. Most 3d programs use the y axis as up/down, but Blender uses Z axis. See if your importer has a setting to make up for that.